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I desperatly need to clean up some geometry that was previously moddeled in a CAD software. Is there a way to automatically select all the red parts so I can filp them? Recalculating normals does not work and selecting all the faces with "L" will take forever since this is not the only object I have to clean up.

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Edit: Select Similar -> Normal also does not work

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  • $\begingroup$ Edit Mode, with the whole mesh selected Recalculate the normals ( ctrl N ) that should fix the problem (from blender.stackexchange.com/a/12174/88382) $\endgroup$ – Sanbaldo Apr 2 at 7:48
  • $\begingroup$ It is possible and likely that surfaces coming from CAD software are not continuous, so Recalculate Normals will not work alone in a lot of cases. $\endgroup$ – Martynas Žiemys Apr 2 at 8:00
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If you select all and merge vertices by distance (alt + m) to avoid separate faces, you can then use Recalculate Normals function (shift + n outside, or shift + ctrl + n inside). If that does not work, it means there is simply no way for the algorithms to determine what the right direction might be - if for example you had a few separate planes in random orientations in the mesh of the object, there would be no way to determine what you want to consider the right direction for normals or even what would be consistent for all of the separate planes for your situation without you specifying that. In that case you need to manually select them and set the normals as you like.

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  • $\begingroup$ Sadly, merging the verts messes up the geometry. Its strange that there is no way to just select all the red faces since blender has already marked them. $\endgroup$ – Luca Apr 2 at 8:02
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    $\begingroup$ It kind of depends on your end goal. What are you looking to do with the model? Merging vertexes in some cases can help but this requires your model to have somewhat clean topology which CAD software is highly unlikely to generate. I'd first recommend checking the model's scale. Then try merging vertexes with a really small merge distance. Lastly you can try to recalculate the normals. $\endgroup$ – Delagone Apr 2 at 12:13
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The best solution I have found is:

  1. Merge Verts
  2. Recalculate Outside
  3. Subdivision Surface to fix broken Geometry
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